Abstract

A kinetic model is devised, from the reaction mechanism steps, to predict the rate of reducing sugar production by hydrolysis of two types of cellulose, namely, amorphous carboxymethylcellulose (CMC) and highly crystalline wood shavings, using Aspergillus niger cellulase. Experimental results in a stirred batch reactor at 40 °C show that the production of reducing sugar reduced at much shorter times for wood shavings in comparison to CMC at the same initial substrate concentration. The experimental results are used to determine the kinetic parameters of the model equations. The significance of crystallinity was determined using inert fraction coefficient, which is assumed to be constant and equals 0.05 and 0.98 for CMC and wood shavings, respectively. It is shown there is a good agreement between the experimental results and proposed kinetic model predictions. The effect of the inert fraction coefficient on the production of reducing sugar by the enzymatic hydrolysis of cellulose is also determined. It is found that the cellulase used extracted from A. niger is much more sensitive towards the substrate structure in comparison to that extracted from Trichoderma reesei.

Keywords: Cellulose; Cellulase; Kinetic model; Inert fraction coefficient; Product inhibition

Nomenclature

d_o

initial average diameter of the substrate particles (m)

[E]

concentration of enzyme (K units m⁻³)

[EP]

concentration of enzyme-product complex (K units m⁻³)

[ES_c]

concentration of enzyme-hydrolysed substrate complex (K units m⁻³)

[ES_x]

concentration of enzyme-nonhydrolysed substrate complex (K units m⁻³)

[E_t]

total enzyme-substrate complex (K units m⁻³)

k₂

rate constant for product formatio (s⁻¹)

k_c1

rate constant for forward formation of active, ES_c (m s⁻¹)

k_c−1

rate constant for backward decomposition of active, ES_c (s⁻¹)

k_p1

rate constant for forward formation of EP complex (m³ kg⁻¹ s⁻¹)

k_p−1

rate constant for backward decomposition of EP complex (s⁻¹)

k_x1

rate constant for forward formation of non-productive, ES_x (m s⁻¹)

k_x−1

rate constant for backward decomposition of ES_x (s⁻¹)

L_o

initial average length of the substrate particles (m)

P

mass concentration of product (kg m⁻³)

[S_c]

hydrolysable substrate surface concentration (m² m⁻³)

[S_t]

total cellulose surface concentration (m² m⁻³)

S_t

mass concentration of cellulose (kg m⁻³)

[S_x]

non-hydrolysable substrate surface concentration (m² m⁻³)

t

time (min)

Greek letters

φ

inert fraction coefficient

ρ

density of the cellulose (kg m⁻³)

Article Outline

Nomenclature

1. Introduction

2. Kinetic model

3. Methods

3.1. Reagents

3.2. Experimental set-up

3.3. Reducing sugars analysis

3.4. Hydrolysis reaction

4. Results and discussion

4.1. Effect of initial substrate concentration

4.2. Effect of substrate quality

5. Model validation

5.1. Determination of kinetic parameters

5.2. Effect of the inert fraction coefficient, φ

5.3. Changes in intermediates concentration

6. Conclusion

References